Hemophilia A and B are bleeding disorders caused by deficiencies in coagulation factor VIII (FVIII) and IX (FIX), respectively. Affected individuals often suffer from spontaneous internal bleedings, the most severe of which could lead to intracranial hemorrhage and death [1]. Current treatments rely heavily on the administration of either plasma-derived or recombinant factor concentrates. However, their usage is limited due to the high cost of recombinant concentrates and the risks of viral transmissions associated with contaminated blood products. Furthermore, it has been estimated that 10-15% of hemophilia A patients and 1-3% of hemophilia B patients develop neutralizing antibodies that render factor replacements ineffective for these patients [2]. It is therefore of great importance to develop new approaches to meet the needs of hemophilia patients.
Several clinical studies have suggested that the severity of bleeding phenotypes in hemophilia patients can be moderated by a fairly common mutation in the factor V (FV) gene known as the factor V Leiden mutation (FVL) [3-7]. Patients carrying this mutation have activated cofactors FVa that are resistant to degradation by activated protein C (APC), a serine protease that normally degrades FVa and FVIIIa [8]. Hemophilia A and B mice carrying the same mutation showed improved hemostasis in vivo, particularly at the microcirculation level, suggesting that APC could be a new target for hemophilia treatment [9]. Over the past few years, several groups have shown that the inhibition of APC by chemically synthesized inhibitors leads to the prolongation of FVa function and is accompanied by increases in thrombin generation and clot weight in hemophilia A blood [10, 11]. These inhibitors, however, have high inhibition constants (Ki values in the micromolar ranges) and are not fully selective against thrombin, which limit their therapeutic usage.